The present invention relates to servo-control devices, in particular for an aircraft flight control actuator.
A preferred application lies with devices for servo-controlling position.
The invention is also advantageously applicable to devices for servo-controlling force.
Conventionally, a position servo-control device for a flight control actuator comprises a servo-control loop which includes compensation means disposed in the loop upstream from the servo-controlled system and performing filtering, in particular at resonant frequencies for movements of the mechanical part(s) actuated by said system.
A problem encountered with such a servo-control device lies in that no account is taken of the forces which act on the mechanical parts being actuated.
Unfortunately, these forces can be particularly large.
In various technical fields, in particular in fields very distant from aircraft flight control actuators, various servo-controlled systems have been proposed for improving control of the force exerted on the mechanical parts that are actuated.
Document EP 0 897 111 discloses a mechanical test machine comprising a servo-control device making it possible to control a load exerted on a sample of material under test. The servo-control device has means for determining an error that is characteristic of the difference between the theoretical reference force and a variable that depends on the real applied maximum and minimum forces.
Document U.S. Pat. No. 5,117,170 describes an exercise machine comprising a servo-controlled motor-driven device serving to simulate a load. In that device, the control signal for the motor depends on the position of the rotor and it cannot exceed a maximum value.
Document U.S. Pat. No. 5,136,227 proposes a servo-controlled device for compensating the effect of gravity on the arm of a robot. That device has means for determining an error characteristic of the difference between the real torque applied and a theoretical torque due to gravity.
Document FR 2 660 705 describes a device for servo-controlling the force exerted on the piston of a hydraulic actuator. That device comprises a servo-control loop that takes account of an error which is characteristic of the difference between the real force applied and a reference force, and an additional loop that takes the speed of the piston into account.
Those various devices do not make it possible to take account of the strength of the mechanical part on which the forces act.
Consequently, it is desirable to provide better control of such forces in order to limit the fatigue of mechanical parts and to increase their lifetime.
An object of the invention is to provide a position servo-control device of this kind which is particularly simple and reliable.
To this end, the invention provides a servo-control device comprising at least one servo-control loop which receives a servo-control signal as input and whose output acts on an element to be servo-controlled, said device comprising means for determining a parameter characterizing the real force generated by the servo-control, the device also including a loop having means for determining an error characteristic of the difference between said parameter characterizing the real force and a parameter characterizing an acceptable theoretical force which is a function of the servo-control signal, and also means for correcting the processing of the servo-control loop as a function of said error.
Such a device, which can be a position servo-control device, or a force servo-control device, presents the advantage of providing very high performance force control.
The device is advantageously associated with the various following characteristics taken singly or in any technically feasible combination:
the means for determining the error comprise determination means for determining a parameter characterizing the acceptable theoretical force on the basis of a reference model;
an actuator controlled by the servo-control loop is of the hydraulic type and the reference model is a force/signal transfer function for controlling flow rate;
the determination means comprise means for implementing a transfer function having the same appearance as a transfer function for determining the parameter characterizing the real force, except in the vicinity of at least one resonant frequency of said force;
said transfer function is:             F      ref              Q      ref        =            ks                                                  (                              1                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      f                    0                                                              )                        2                    ⁢                      s            2                          +                  2          ⁢          x          ⁢                      𝒳                          2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                0                                              ⁢          s                +        1              ·          B      ⁢              (        s        )            
where s represents the Laplacian, and where f0 is equal to or close to a resonant frequency, "khgr" being an attenuation factor having magnitude of about 0.7 to 1, k being a gain factor, B(s) being a filter function; and
the function B(s) is equal to 1.
The invention also advantageously provides a hydraulic servo-control device constituted by a device of the above-specified type.
The invention also provides a servo-control device for a flight control actuator or a test actuator, the device being constituted by a device of the above-specified type.